One convenient way to join lengths of pipe end-to-end is to form a bell-shaped socket on one end of each piece of pipe. The normal or unbelled end of each piece slips into the socket formed by the belled end of an adjacent piece, thereby providing a convenient manner of forming a pipeline in the field.
Thermoplastic pipe is extruded continuously and cut into predetermined lengths while traveling from the extrusion apparatus. The desired time for forming a socket or bell at one end of each pipe length is at the end of the extrusion line, where the newly-formed pipe is still hot. Pipe belling is conventionally accomplished by inserting a mandrel into the end of the pipe to form the bell, and additional heating may be required to bring the pipe end to a suitable forming temperature. Once the mandrel is inserted, the pipe is cooled to reduce its plasticity and finally the mandrel is removed.
Various machines have been built or proposed to perform the foregoing bell forming operations. The bell or socket to be formed is tapered to provide the desired end-to-end fit with an adjoining length of pipe, and to meet standard requirements the bell must be held to close tolerances on the entrance and bottom dimensions. These tolerances require accurate and controlled depth penetration of the mandrel into the pipe end being belled.
Various techniques for controlling the depth of the pipe bell or socket have been proposed in the art, with less than satisfactory results. Some methods have assumed that the pipe being belled is the same length each time, which in practice can vary. Other depth controlling techniques call for positioning the end of the pipe at a predetermined point, and then gripping the pipe while the mandrel is moved forwardly a certain distance relative to the predetermined point. This technique does not allow for pipe slippage in the gripping device during the belling operation, or for mispositioning of the previously-aligned pipe by the gripping device itself.